1. Field of the Invention
The present invention relates to a semiconductor device, a method for fabricating the same, and a transformer circuit using the same, and more particularly, to a semiconductor device capable of achieving an enhancement in fabrication efficiency, a reduction in fabrication costs, and an enhancement in operation reliability, a method for fabricating such a semiconductor device, and a transformer circuit using the same.
2. Discussion of the Related Art
A switching mode power supply (SMPS) is used as a DC-stabilizing power source for an electronic communication appliance such as an electronic calculator or an electronic switching system. The SMPS controls a flow of electric power, using the switching process of a semiconductor device. Accordingly, the SMPS has superior advantages in terms of high efficiency, compactness, and lightness, as compared to conventional stabilizing power sources. In this regard, the SMPS is widely used as a stabilizing power source.
It is well known that, in electronic communication appliances, systems have rapidly advanced in terms of compactness and lightness in accordance with the development of semiconductor integrated circuits, but the power source(s) thereof still have limitations on compactness and lightness due to the use of an inductor and a capacitor as passive elements for energy storage. As a result, the compactness and lightness of the SMPS may be relatively more important than the system components for the compactness and lightness of the electronic communication appliance.
The SMPS may be mainly classified into AC-DC types, in which an AC voltage is converted into a DC voltage, and DC-DC types, in which a first DC voltage is converted into a second DC voltage having the same characteristics as the first DC voltage. The SMPS may also be classified into an insulation type and a non-insulation type. The insulation type SMPS may include a buck or step-down type, a boost or step-up type, and a buck-boost type. The insulation type SMPS generally includes a buck-boost type converter (e.g., a flyback converter or a ringing chock converter [RCC]) or a buck type converter (e.g., a forward type converter, a half bridge type converter, a full bridge type converter, or a push-pull type converter). The converter mainly uses a transformer.
The boost type converter is a DC-DC boost converter, which boosts an input DC voltage, and outputs the boosted DC voltage. The DC-DC boost converter controls the switch-on time of a transistor, using a boost controller, and thus controls an output voltage compared to an input voltage.
FIG. 1 is a circuit diagram illustrating a general DC-DC transformer circuit including a boost controller. Referring to FIG. 1, the DC-DC transformer circuit 10 includes a power source 40 for supplying an input voltage for the DC-DC transformer circuit 10, a MOS transistor 30 for switching the load of the current supplied from the power source 40, and an inductor 42 for accumulating or discharging energy supplied from the power source 40 in accordance with the switching operation of the MOS transistor 30. The DC-DC transformer circuit 10 also includes a diode 44 for preventing the current from flowing backwards in the circuit 10, a capacitor 46 for outputting the energy discharged from the inductor 42, and a boost controller 20 for controlling an ON-OFF timing of the MOS transistor 30.
The MOS transistor 30 has a structure corresponding to a field effect transistor (FET) in which a metal or polysilicon gate is isolated from a semiconductor (source/drain). Hereinafter, the MOS transistor 30 will be referred to as a “MOSFET”. The MOSFET 30 includes a source, a gate, and a drain, respectively corresponding to an emitter, a base, and a collector in a bipolar transistor or bipolar junction transistor (BJT). In the general DC-DC transformer circuit 10 shown in FIG. 1, the MOSFET 30 is turned on or off by the boost controller 20, to function as a switching element to change the load of the transformer circuit 10.
The boost controller 20 controls the ON-OFF timing of the MOSFET 30, namely, the operation time of the MOSFET 30, using a periodic signal from an oscillator.
In the DC-DC transformer system using the boost controller 20, energy supplied from the power source 40 is accumulated in the inductor (L) 42 when the MOSFET 30, which functions as a switch, is in an ON state. When the MOSFET 30 is subsequently turned off, the energy (current) accumulated in the inductor 42 is discharged to an output stage, namely, the capacitor 46, via the diode 44. In this boost transformer system, the voltage supplied from the power source 40 is outputted at a maximum value from the capacitor 46 when the OFF time of the MOSFET 30 is minimal. As the ON time of the MOSFET 30 is varied, the value of the boosted voltage is adjusted. The boost controller 20 is referred to as a “boost converter” or a “step-up converter” because the boost controller 20 has operation characteristics in which the output voltage has a value generally higher than that of the input voltage.
The general DC-DC transformer circuit 10 has a drawback in that a large space for the circuit configuration thereof is needed because the MOSFET 30 and boost controller 20 are configured by separate integrated circuits, respectively. Since such separate integrated circuits are used, there is also a drawback in that the fabrication time and costs of the DC-DC transformer circuit 10 increase. Furthermore, a large number of elements are combined to fabricate the DC-DC transformer circuit 10. For this reason, there is a drawback in that the reliability of the output voltage is lowered due to a possible malfunction of the elements.